1. Field of the Invention
This invention relates to an air filter. It relates particularly to an air filter for removing particulate matter, water and oil, whether in the form of vapour or droplets, from compressed air systems such as air braking systems.
2. Description of the Prior Art
Hot compressed air has a greater propensity for carrying water and oil vapour and suspended water and oil droplets and other particles than has non-compressed room temperature air. It is therefore frequently important for filtering apparatus to be provided for removing contaminants from a system which utilizes compressed air.
Conventional systems for removing contaminants have involved cooling by means of an air cooler which encourages precipitation of water and oil vapours, followed by passage through a porous filter medium which removes suspended water and oil droplets and other foreign particles. However, the air coolers used in such systems have generally been large and expensive, and the porous filter medium has generally become blocked in a short period of time so that frequent maintenance or replacement has been required.
Over the past few years, more efficient means of removing vapours from compressed air have been developed. Examples of these new types of humidity removers are given in U.S. Pat. No. 4,662,907, International Publication No. WO88/02658 and U.S. Pat. No. 4,848,988. Each of these specifications describes a device having an inner cylinder completely encompassed within an outer cylinder. Compressed air is injected through flared nozzles located near the top of the inner cylinder, and the inner cylinder functions as an expansion chamber in which the air expands rapidly and so cools adiabatically. Cooling results in precipitation of vapours, and the devices described in U.S. Pat. No. 4,662,907 and International Publication No. WO88/02658 provide filter nets across the inner chamber to trap droplets so precipitated. The device disclosed in U.S. Pat. No. 4,848,988 relies on a baffle plate to trap precipitated droplets. The water and oil then flows down the sides of the inner chamber and accumulates at the bottom of the outer chamber, from which it may be removed by an outlet port. The dried air passes from the bottom of the inner chamber into the outer chamber and out through an outlet port at the top of the outer cylinder.
While these types of filters are effective in removing oil and water from compressed air, it has been found that after a short period of use the flared injection nozzles located at the top of the inner cylinder frequently become blocked by particles, oil and moisture, so that regular servicing is required.